CNC facing lathes are widely applied in the workpiece machining field, because its radial slide can be moved conveniently on the facing head. On existing CNC facing lathes, the slide is driven by a rack-and-gear mechanism to slide in radial direction, i.e., the main rack runs across the spindle and is driven by an electric motor behind the lathe ram via the screw shaft, the other end of the main rack drives a gear in the facing head to rotate, and the gear drives a rack on the slide to move, so that the slide moves in radial direction. However, that driving structure has some major drawbacks: first, the positioning repeatability is low and the machining efficiency and workpiece machining accuracy are degraded, due to the severe backlash between the rack and the gear during power transfer; second, the facing head system will be out of balance when the feed slide biases to a side; consequently the facing head will have vibration or shock when it rotates at a high speed, resulting in degraded work piece machining accuracy or limited rotation speed of the facing head; third, the mechanism has a complex structure and has a strict requirement for assembling accuracy.